1. Field of the Invention
The present invention is concerned with decreasing bromine-reactive contaminants in hydrocarbon feeds. More specifically, the present invention is concerned with a process for removing bromine-reactive contaminants from hydrocarbon feeds that has a longer cycle length and utilizes a crystalline molecular sieve catalyst.
2. Description of the Prior Art
Hydrocarbon feeds, such as aromatic hydrocarbon feeds, are derived from processes such as naphtha reforming and thermal cracking (pyrolysis) and can be used as feedstocks in a variety of petrochemical processes, such as para-xylene production from an aromatic hydrocarbon feedstock containing benzene, toluene and xylene (BTX), toluene disproportionation, xylene isomerization, alkylation and transalkylation. However, aromatic hydrocarbon feedstocks often contain contaminants comprising bromine-reactive compounds including unsaturated hydrocarbons, such as mono-olefins, multi-olefins and styrenes, which can cause undesirable side reactions in these downstream processes. Therefore, these contaminants should be removed from the aromatic hydrocarbon feedstocks before they can be used in other processes.
Improved processes for aromatics production, such as that described in the Handbook of Petroleum Processing, McGraw-Hill, New York 1996, pp. 4.3-4.26, provide increased aromatics yield, but also increase the amount of contaminants. For example, the shift from high-pressure semi-regenerative reformers to low-pressure moving bed reformers results in a substantial increase in bromine-reactive components in the reformate streams, which are aromatic hydrocarbon feedstocks for downstream processes. This, in turn, results in a greater need for more efficient and less expensive methods for removal of bromine-reactive contaminants from aromatic hydrocarbon feedstocks, e.g., reformate streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). The number of grams of bromine absorbed by 100 grams of a hydrocarbon or a hydrocarbon mixture indicates the percentage of double bonds present. Thus, when the type and molecular weight is known, the contents of the olefin can be calculated. The Bromine Indices (i.e., numbers) of the hydrocarbon feeds and products are measured to determine the change in composition. Molecular sieves and clay treating have been used to reduce the Bromine Indices of various hydrocarbon products.
The treatment of hydrocarbons to remove olefinic materials from the hydrocarbons using clay catalysts is widely practiced in the petroleum and petrochemical industries One of the most common reasons for this treatment is to remove olefinic materials in order to meet various quality specifications. As used herein, the term “olefinic material” or “olefinic compound” includes both mono-olefins and multi-olefins. The term “mono-olefins” means olefinic compounds containing one carbon-carbon double bond per molecule. Examples of mono-olefins are ethylene, propylene, butenes, hexenes, and octenes. The term “multi-olefins” means olefinic compounds containing at least two carbon-carbon double bonds per molecule. Examples of multi-olefins are butadienes, cyclopentadienes, and isoprenes.
Olefinic compounds may be objectionable in aromatic hydrocarbons at even very low concentrations of less than a few parts per million by weight (wppm) for some processes. For example, in the manufacture of nitration grade aromatics including benzene, toluene and xylenes, it is essential to remove these olefinic materials from the feedstock.
Molecular sieves have been recently proposed as catalysts for removal of olefinic materials from hydrocarbon feedstocks. For example, U.S. Pat. No. 6,368,496 involves the removal of olefinic materials from an aromatic feed using an acid-active catalyst, such as a catalyst comprising a crystalline molecular sieve with ring structures of ten to twelve members or greater.
Molecular sieves catalysts have certain advantages over clay catalysts in the removal olefinic material from hydrocarbon feeds. For example, molecular sieves catalysts usually have a longer operating cycle than clay catalysts, which results in fewer catalyst change-outs and resultant equipment downtime. In addition, the longer operating cycle of molecular sieve catalyst results in the disposal of less catalyst waste. On the other hand, molecular sieve catalysts are considerably more expensive that clay catalysts. Therefore, it is important for economic viability, that molecular sieve catalysts used in the removal of olefinic material from hydrocarbon feeds, have good activity maintenance.
The present invention is directed to a process for removing bromine-reactive components from a hydrocarbon feed that utilizes a crystalline molecular sieve catalyst and has improved cycle length.